Influence of molecular weight on fatigue behavior of polyethylene and polystyrene

Abstract

Fatigue tests in reversed tension‐compression have been carried out on samples of polyethylene and polystyrene of widely varying molecular weights, extending up to 2, 000, 000. All tests on polystyrene specimens were made at 1600 rpm. For polyethylene, tests speeds had to be reduced to 100 rpm in order to avoid serious temperature effects. For both materials, increasing molecular weight leads to improved resistance to alternating loading. For polystyrene, this improvement in ultimate properties even continued well beyond molecular weight values where Tg, becomes effectively independent of molecular weight. For polyethylene, samples of high molecular weight did not fail even after 107 cycles of alternating loading at a stress level of 3000 psi.

An Analytical Method To Predict Fatigue Life of Thermoplastics in Uniaxial Loading: Sensitivity to Wave Type, Frequency, and Stress Amplitude

“…24,27 The frequency applied also appears to have an effect on the time-to-failure of the signal with a stress amplitude of 10 MPa, in particular at higher frequencies (see Figure 16a). It is well-established that polymers dissipate a substantial amount of energy at high stress [48][49][50][51][52][53] and high frequency, [53][54][55][56] which causes substantial heating of the sample as also demonstrated in ref 13. For that reason, a frequency dependence may be anticipated at higher amplitudes, where hysteretic heating is more likely to occur.…”

Fatigue of polymers

“…In one study carried out at a low frequency of less than 2 Hz so that thermal effects would not be a serious problem, it was observed [14] that, for all specimens having a molecular weight of 50,000 or over, no fatigue failures occurred in less than 5 million cycles, even though the applied alternating tension-compression stress was 20.6 MPa, a value only about 10% less than the yield stress in simple tension. For comparison, under similar stress condition, the glassy polymer PS-even though its tensile strength is almost double the yield strength of PE-develops fatigue fracture in about 104 cycles [15].…”

“…Many investigators have, for given polymers, provided data as to its magnitude and influence. Figure 1 shows that the specimen temperature, for polyethylene (PE) unnotched samples tested in uniaxial loading increases linearly with the test frequency and, for any given frequency, it increases even more rapidly with increase of the applied alternating stress [14]. For polystyrene (PS) -which has a very low internal friction-it is possible to run similar fatigue tests at 1600 cpm and with a TEST SPEED (cpm)…”

“…This type of behavior, for example, is illustrated in Fig. 5 for a commercial PS [15], of broad molecular weight distribution (MWD), and for three narrow MWD PS's of varying molecular weight value [14]. As the stress amplitude is further decreased the o--N curve tends to become flat and a limiting stress, the so-called endurance limit, can then be defined as that stress below which no fatigue Figure 5.…”

Effect of section thickness on fatigue performance of laser sintered nylon 12

“…Amel et al [7] investigated fatigue behaviour of LS Nylon 12 parts subject to displacementcontrolled tension only fatigue loading. In a recent study, Van Hooreweder et al [8] have explored the difference in fatigue behaviour of LS and injection moulded Nylon Several factors, both internal and external, have been identified as parameters that could influence cyclic performance and fatigue life of polymers [9]- [11]. Therefore, any parameter that is considered likely to affect the internal structure of the polymer parts might influence the cyclic performance as well.…”